The present invention relates to microsystems and methods for producing microsystems. In particular, the present invention relates to flexible Microsystems which can be given almost any desired shape.
Due to the very fast progress in the field of silicon technology, whole Microsystems are integrated in silicon in an increasing number of cases. These modern microsystems include in addition to the evaluation electronics for a sensor arrangement also the sensors and actuators themselves, which are also formed e.g. in a silicon substrate by means of micromechanical technologies. At present, such Microsystems are produced from monocrystalline-silicon substrates in most cases. The electric properties of monocrystalline silicon permit the integration of such complex circuits. For realizing sensor elements and actuator elements, the mechanical properties of the silicon as a material itself are utilized in addition to the electric properties. Other electronic materials, such as amorphous or polycrystalline silicon and also conductive polymers, which can be used alternatively, are far from achieving an electric behavior comparable to that of monocrystalline silicon.
Due to the high mechanical hardness of silicon, substrates used for known Microsystems are of solid silicon and therefore rigid and planar. The wafer thickness is in the order of a few hundred micrometres. This is the reason for the fact that such microsystem chips are not suitable to be used in surroundings with uneven support surfaces. The disadvantage of the lack of mechanical flexibility becomes increasingly apparent as the size of the chip area increases. It should be noted that complex microsystems may occupy chip areas of a considerable size, taking into account that, in addition to an integrated circuit, also the sensor and the actuator themselves are arranged on the wafer.
A known approach to increasing the flexibility of microsystems is described in Trieu, H. K., Ewe, L., Mokwa, W.: xe2x80x9cFlexible Mikrostrukturen in Siliziumxe2x80x9d, Fraunhofer IMS Jahresbericht 1996, pp. 17-19. For solving the flexibility problem, mechanically flexible MESAflex substrates are suggested consisting of a silicon island array with silicon islands which have a width of 0.8 to 2 mm and a thickness of 0.3 to 0.7 mm and which are interconnected via thin silicon membranes (thickness less than 20 xcexcm). In comparison with conventional microsystems, which are integrated on a single silicon wafer with a thickness of a few hundred micrometres, such MESAflex substrates are characterized by a much higher flexibility. Arrays of such silicon islands permit linear deflections of more than 90xc2x0 out of the principal plane.
However, these monocrystalline silicon membranes, which interconnect the silicon islands, are still too sensitive and fragile for many cases of use. Furthermore, the local flexibility, i.e. the flexibility in a certain area of the microsystem, is limited by the comparatively large silicon islands.
It is the object of the present invention to provide microsystems which are reliable and, nevertheless, highly flexible.
In accordance with a first aspect of the present invention. this object is achieved by a microsystem comprising a flexible foil; a plurality of semiconductor elements embedded in said flexible foil, said flexible foil supporting the semiconductor elements without any rigid support being provided; and at least one connection line arranged substantially on said flexible foil and used for electrically connecting at least two semiconductor elements.
In accordance with a second aspect of the present invention, this object is achieved by a method for producing a microsystem comprising integrating electronic components in a semiconductor layer, which is provided on a wafer surface in such a way that at least two electronic components are arranged at locally different positions in the wafer; etching the areas between said electronic components free whereby semiconductor elements connected to the wafer and projecting beyond said wafer are produced; applying a flexible foil to the wafer surface in such a way that the semiconductor elements are embedded in said flexible foil; applying a connection line to the flexible foil in such a way that said connection line is connected to contact areas of semiconductor elements; and etching the wafer away from the back of said wafer in such a way that the semiconductor elements are no longer mechanically interconnected by said wafer and said semiconductor elements are held by said flexible foil without any rigid support being provided.
A microsystem according to the present invention comprises a flexible foil, a plurality of semiconductor elements embedded in this flexible foil, and at least one connection line arranged substantially on this flexible foil and used for electrically connecting at least two semiconductor elements. The characteristic features with regard to which such a microsystem according to the present invention differs from the prior art are that semiconductor elements, which may be integrated circuits and also sensors or actuators, are no longer interconnected via a wafer or via silicon membranes. The use of a wafer, which mechanically interconnects all the individual semiconductor elements and which turns the microsystem into a rigid or, when silicon membranes are used, comparatively brittle object, can be dispensed with due to the use of a flexible foil having the semiconductor elements embedded therein. The individual semiconductor elements are only supported by the flexible foil according to the present invention. Hence, they are also insulated from one another, when the flexible foil is an insulating foil. According to the present invention, the flexible foil can then have applied thereto the necessary structure of conducting tracks and bus lines so as to interconnect the individual semiconductor elements for obtaining a desired microsystem.
The semiconductor elements can, quite generally, be regarded as flat rectangular parallelepipeds which can be embedded in the flexible foil in several ways. On the one hand, the semiconductor elements, which have only thicknesses of a few micrometres down to the submicrometre range, can be embedded in the flexible foil in such a way that both main surfaces of the semiconductor elements having the shape of flat rectangular parallelepipeds are exposed. A connection line can then be applied directly to connecting areas of a semiconductor element. Preferably, the very thin semiconductor elements are, however, embedded in a foil in such a way that only the back of the semiconductor elements is exposed. The front of the semiconductor elements having the connecting areas arranged thereon is then also covered by the flexible foil. For producing connection lines, it will then be necessary to provide via holes in the flexible foil so as to connect and interconnect the semiconductor elements.
According to the present invention, it is essentially the flexible foil that interconnects the individual semiconductor elements mechanically. The flexibility of the microsystem is therefore no longer determined by the rigid semiconductor elements, but essentially by the flexible foil. Further advantageous possibilities exist for adapting the flexibility of the microsystem to specific circumstances, i.e. for adjusting a local flexibility of, the microsystem. By arranging the semiconductor elements relative. to one another in a specific manner, e.g. in functional blocks, it is possible to provide the microsystem with a flexibility of such a nature that the microsystem can be folded or rolled. Another advantage of the present invention is that specific areas of the flexible foil can be shaped by means of xe2x80x9creinforcingxe2x80x9d bridges which are adapted to be geometrically structured in an arbitrary manner so as to provide e.g. a comparatively stiff microsystem in the x-direction, but a very flexible microsystem in the y-direction. It follows that, just as a specific arrangement of the semiconductor elements in the flexible foil, this measure permits the production of a local stiffness or of the contrary, viz. a local flexibility of the microsystem.
Furthermore, it is possible to influence the properties of the flexible foil directly, i.e. when the flexible foil used is a ploymer foil, the stiffness of the foil can be adjusted locally by different curing conditions.
In comparison with known flexible circuits, in the case of which semiconductor chips are applied to a flexible substrate by means of soldering or by means of an adhesive, the microsystem according to the present invention, in the case of which the semiconductor elements are embedded in the flexible foil, offers the advantage that loads caused when the microsystem is being bent are taken up substantially completely by the flexible foil but not by the semiconductor element, whereby mechanical stresses occurring when the microsystem is being bent will not result in piezoresistive influences on the electric parameters of the semiconductor elements. Although the flexible microsystem as a whole can be adapted to almost any shape or can be given almost any shape, the individual semiconductor elements as such will maintain their individual structure, i.e. they will not be bent, and this is particularly important in the case of certain sensors or actuators whose mechanical shape must not be xe2x80x9cdistortedxe2x80x9d.